The advent of low floor transit vehicles fueled by alternative fuels such as compressed natural gas (CNG), liquefied natural gas (LNG) or hydrogen, has resulted in the need to mount fuel storage means in the form of pressurized vessels to the vehicle.
Typically, in order to achieve a driving range similar to a diesel vehicle and to achieve safety standards associated with alternative fuels, a plurality of pressure vessels must be used. In order to reduce the weight of such fuel storage systems, lightweight composite pressure vessels and mounting systems are used.
In order to meet ANSI/AGA NGV2 and CSA B51 certification in both the U.S. and Canada, the mounting systems must be designed to accommodate radial and axial growth of the fuel pressure vessels as a result of pressurization thereof. Further, the pressure vessels must withstand dynamic loading. The dynamic loads may be specified in terms of multiples of gravity. The loading design is dependent on the orientation of the vessel. In Canada, where pressure vessels are typically oriented in the same direction as travel of the vehicle, the design dynamic loading must be at least 20 g in the longitudinal direction of the vehicle and 8 g in any other direction. These loads supersede those required for normal operation and are generally more stringent than those imposed in the U.S., where vessels are oriented in the same direction. Further, a maximum allowable deflection of 0.5 inches (12.5 mm) for mounting brackets is required when tested at 8 g. When pressure vessels are mounted crosswise to the direction of travel, such as is the convention in Europe and Japan, the current design crash loads are 100 g in all directions. The standards periodically change.
In 1998, Lincoln Composites (Lincoln, Nebr., U.S.A.), a division of Advanced Technical Products, Inc., disclosed a modular concept for roof mounting utilizing a lightweight truss frame, expandable to accommodate various lengths of pressure vessels. Integration of the modules to a bus roof is accomplished by utilizing mounting brackets that can be relocated along the length of the modules to correspond with the roof “hard points” or rigid frame structure. The modular frame includes a plurality of end members disposed between two longitudinal rails and a plurality of truss-like central frame members disposed parallel to the longitudinal rails. The pressure vessels are positioned lengthwise in parallel with the central frame members, thereby separating the pressure vessels from each other and adding structural rigidity to the modular frame.
Other frames have been designed to meet safety requirements and weight restrictions. One such known design is typically used for roof-mounting in low floor buses comprising a frame structure of end members and cross members. The frame has steel straps at two places along each pressure vessel, clamping each pressure vessel into the frame.
In the Lincoln Composites system described above, pressure vessels are positioned with a longitudinal axis oriented in the same direction as the longitudinal axis of the vehicle. In other known frames, pressure vessels are oriented with the longitudinal axis at 90 degrees to the frame rails and the longitudinal axis of the vehicle. The differences in orientation of the pressure vessels are representative of differences in mounting conventions between North America and those in Japan and Europe.
It would be desirable to have a mounting system and a method for mounting a pressure vessel, wherein the system and method maximize a vessel capacity and dynamic load performance.